"The best way to prevent influenza is by getting vaccinated each year," Dr. Karen Midthun, director of the FDA's Center for Biologics Evaluation and Research, said in a statement. "It is especially important to get vaccinated this year because two of the three virus strains used in this season's influenza vaccines differ from the strains included in last year's vaccines."

Experts from the FDA, the U.S. Centers for Disease Control and Prevention and the World Health Organization, along with other public health officials, study global flu infection patterns to try to predict which strains of the virus are most likely to make people ill in the coming flu season. Sometimes there is a mismatch between the strains in the vaccine and the strains that wind up infecting most people each season, but officials noted that even then the vaccine can lessen the severity of illnesses.

Six vaccine makers are licensed to provide the flu vaccine in the United States, and the 2012-2013 vaccine will include the following strains: A/California/7/2009 (H1N1)-like virus; A/Victoria/361/2011 (H3N2)-like virus; and B/Wisconsin/1/2012-like virus. The H1N1 virus is the same as the one included in last year's vaccine, while the other two differ from the strains used in the 2011-2012 vaccine, according to an FDA news release.

Between 5 percent and 20 percent of the U.S. population develops influenza each year, resulting in more than 200,000 hospitalizations from related complications. Influenza seasons can vary widely, with annual influenza-related deaths ranging from a low of about 3,000 to a high of about 49,000 people in the United States, the news release stated. The CDC recommends that everyone 6 months of age and older receive an annual influenza vaccine.

The companies producing the 2012-2013 flu vaccines and the brand names of the vaccines are: